So far I've already played around with the pitches.h library and hooked up a small 8ohm 0.1w speaker directly to the arduino (using a 150ohm resistor too) So far that has worked fine with my program.

But my goal for part of my project is to hook up a larger more powerful speaker (I have a 4" 8ohm 10w or a 2" 8ohm 3w speaker that I'd prefer to use) and get loud audio (simple tones really). So, I need some kind of amplifier.

I've started to search around the forum and on google but I just get more confused as I look at amp circuits. I tried making the audio amplifier from the Aduino Basic Connections guide http://arduino.cc/forum/index.php/topic,154549.0.html Although I didn't have a BC337 on hand. I used a P2N2222 instead. (I also have some BC328, BC549, and BC550 lying around) I did get some audio out of the speaker, but it isn't what I'd call loud.

I also tried making this: (found it searching google)but that was even worse, as I didn't get any audio from the speaker.

Poking around I do see CrossRoad's circuit using IRF3707Z MOSFET http://arduino.cc/forum/index.php/topic,157647.msg1184322.html#msg1184322, but I'd have to order one (would prefer to use components I already have if possible).

I also see this post http://arduino.cc/forum/index.php/topic,114094.msg858503.html#msg858503 but there is no guidance on what values are needed for the resistors and capacitors. I would try making it, but what components do I use? (I have a fairly good assortment of resistors/capacitors and the 4 transistor types I mentioned) I'll also be powering my project from a 12v battery, so would prefer to use that as the voltage level for the speaker (but could just as easily step it down, since I have a 12v-5v switching voltage regulator)

the 2222 is not that great for amplification applications, have you tried your other transistors? (look at the datasheets, like the 328 is a PNP, the 549 and 550 are npn)

and loud is depending on the source, some hard nasty square waves though a single transistor amp is going to be brutal, music on the other hand may not be as noticeable.

personally I am not that good with amp's, I just kind of throw crap at it until its to my tastes (in the case of your third circuit), both cap's are electrolytic, c1 blocks DC and depending on what size it is will cut off frequencies, not entirely sure what C2 does but its probably for clarity of some form

If you just want to amplify tones generated by the Arduino, try the attached schematic. It will deliver about 0.3W into 8 ohms. If you need more volume than that provides and you want to run the amplifier from 5V, then I suggest this chip http://www.ti.com/lit/ds/symlink/lm4871.pdf or someting similar.

If you just want to amplify tones generated by the Arduino, try the attached schematic. It will deliver about 0.3W into 8 ohms. If you need more volume than that provides and you want to run the amplifier from 5V, then I suggest this chip http://www.ti.com/lit/ds/symlink/lm4871.pdf of someting similar.

It's simple because it only has to amplify square waves of a fixed amplitude, which is what comes out of an Arduino digital pin or PWM pin. It's no good for amplifying general audio signals, or the outputs from the DACs on the Due.

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Surely all you need to do is treat your speaker like any other PWMed load selecting appropriate voltage to not blow the speaker. You could use a low pass filter to try and round off the square wave but that could get tricky if your using different frequencies

I use LM386N8's, easy to put down on either perf board or dead bug and DIP8 chip's are available... Just don't hit it with too much audio drive. The fet amp with the 2SK part would be best described as self biased class A (Gate bias is derived from the drain current only). It also draws an inordinate amount of current for what it needs to do.. These guys are semi local to me and the prices are good too http://www.oddwires.com/chip-amps/ I always keep 3 or 4 of the LM386's just for this reason... One unusual use is to make the chip oscillate and rectify the output for a neg bias supply with some current available. All in all, Low voltage use, gains from 20 to 200 and operation from 4 to 12V (use 9V or less, 5V is OK but NEEDS a big supply bypass capacitor (220 to 470 uF) make it a fine choice. For many years I used one as an audio signal tracer and many times was very handy for digital work as well. Use your imagination.

I studied some electronics YEARS ago so I can get a feel for some of his design but I can't see what is gained by dropping half the power across a 40W resistor.

Advantages of class-A Audio amplifiers

Class-A designs are simpler than other classes; for example class-AB and -B designs require two devices (push-pull output) to handle both halves of the waveform; class A can use a single device single-ended.

The device is never shut off completely there is no "turn on" time, little problem with charge storage, and generally better high frequency performance and feedback loop stability (and usually fewer high-order harmonics).

The point at which the device comes closest to being cut off is not close to zero signal, so the problem of crossover distortion associated with class-AB and -B designs is avoided.

Disadvantage of class-A audio amplifiers

They are very inefficient. A theoretical maximum of 50% is obtainable with inductive output coupling and only 25% with capacitive coupling.